1. Field of the Invention
The present invention relates to internal combustion engines with exhaust gas recirculation (EGR) systems and the measurement of EGR flow.
2. Description of the Related Art
One of the essential elements in the over 30-year quest for reducing emissions in the United States has been the use of exhaust gas recirculation or EGR. This is a way in which a selected percentage of products of combustion are recirculated to the intake of an air-breathing, fuel-consuming internal combustion engine to lower combustion temperatures and thus reduce the quantity of oxides of oxygen produced during the combustion process. EGR was initially used on spark ignition gasoline fuel automotive engines. The early EGR approaches, while suppressing oxides of nitrogen, added complexity to the system and otherwise compromised performance and flexibility of the engine with which it is associated.
In succeeding years, the use of EGR has been made significantly more sophisticated with the use of electronic control, oxygen sensors, and other means for closed-loop control of the EGR. In recent years, the application of EGR has been required for compression ignition, or diesel, engines owing to mandatory Environmental Protection Agency (EPA) required reductions in emissions. The application of EGR to diesel engines posed a new challenge for the design of systems, especially in view of the nature of the duty cycle for the diesel engine. Diesel engines, particularly in the off highway field, operate under extremely difficult ambient environments and require control systems for the EGR to be rugged enough to withstand the required engine duty cycle but sufficiently accurate to maintain the required levels of emissions.
In order to control the percentage of EGR relative to the fresh air introduced to the engine, a measurement of EGR flow is required. Currently, such measurement involves either a venturi or orifice positioned within the passage that carries EGR from the exhaust of the engine to the engine intake. While providing some measure of EGR flow, the nature of the gas constituent in the EGR passage is such that it can affect the accuracy of the measurement. Furthermore, the EGR flow passage is subject to significant pressure pulse variations owing to the fact that multicylinder reciprocating internal combustion engines generate discrete exhaust pulses which are reflected in the flow through the EGR passage. This, in turn, can have a significant and adverse effect on the measurement of EGR flow within the EGR passage. In addition to this problem, the use of a venturi or orifice-like restriction in the EGR passage is a direct fuel efficiency penalty since additional pumping work must be done by the engine because of the orifice or venturi restriction.
Accordingly, what is needed in the art is a more effective and accurate measurement of EGR in an internal combustion engine.